Organophosphorus compounds are chemical compounds which comprise primarily the elements carbon, hydrogen and phosphorus. Optionally they also comprise oxygen and nitrogen. In addition, the organophosphorus compounds can comprise further elements such as halogens, sulfur etc. in the form of substituents. The term here includes both those organophosphorus compounds which have a P—C bond, as well as those without P—C bond, i.e. with a P—O or a P—N bond.
Organophosphorus compounds have gained considerable industrial significance because of their wide range of use. They are used directly as plasticizers, flame retardants, UV stabilizers or as antioxidants. In addition, they are important intermediates in the production of fungicides, herbicides, insecticides and pharmaceuticals.
A specific field of use of the organophosphorus compounds is catalysis:
For instance, especially phosphines, phosphites and phosphoramidites are used as ligands in catalyst complexes, which are used in turn for the homogeneous catalysis of processes operated on an industrial scale. Particular mention should be made of the hydroformylation of unsaturated compounds with carbon monoxide and hydrogen, which generally takes place in the presence of a homogeneous catalyst system which has a metal and at least one organophosphorus compound as ligand. An introduction to homogeneously catalysed hydroformylation is given in:    B. CORNILS, W. A. HERRMANN: Applied Homogeneous Catalysis with Organometallic Compounds. Vol. 1 & 2, VCH, Weinheim, N.Y., 1996    R. Franke, D. Selent, A. Borner: Applied Hydroformylation. Chem. Rev., 2012, DOI:10.1021/cr3001803
The synthesis of phosphorus ligands is described repeatedly in the literature. A good overview can be found in    “Phosphorous(III) Ligands in Homogeneous Catalysis—Design and Synthesis” by Paul C. J. Kamer and Piet W. N. M. van Leeuwen; John Wiley and Sons, 2012.
In the synthesis of these ligands, chlorine-containing reagents are frequently used. For instance, in the synthesis of phosphite ligands, phosphorus trichloride (PCl3) is usually used.
The chlorine compounds used in the preparation of organophosphorus compounds present many difficulties in the proper use or further processing of the organophosphorus compound:
For instance, the desired organophosphorus compound is never obtained in pure form immediately, and is always obtained in contaminated form as an organophosphorus product which, as well as the desired organophosphorus compound, also contains contaminants. The contaminants are unconverted or incompletely converted reagents, auxiliaries or products from side reactions. In this context, contaminants in the form of chlorine compounds present particular difficulties:
If the chlorine-containing contaminants get into a steel pressure reactor together with the organophosphorus compound used as ligand, the pressure reactor is subject to increased corrosion as a result of the chloride. This is especially true of continuous processes, in which the organophosphorus compounds are metered in over the course of the reaction. This is the case, for example, when the organophosphorus compound is used as a ligand in industrial scale hydroformylation. The metered addition inevitably also results in an accumulation of the secondary components in the reactor. This is particularly critical if chloride is one of the secondary components since chloride attacks even stainless steels:    Merkblatt [Information sheet] 893 “Edelstahl rostfrei für die Wasserwirtschaft” [Corrosion-free stainless steel for water management], 1st edition 2007, publisher: Informationsstelle Edelstahl Rostfrei, Dusseldorf.
In the presence of chloride ions, there is a particular risk of stress-cracking corrosion, which can lead in more favourable cases to a premature shutdown of the process and to a reactor overhaul, but in less favourable cases even to rupture of the reactor. It is therefore of overriding importance to prevent entrainment of chlorine-containing compounds via the organophosphorus catalyst system.
The ready-to-use phosphorus ligands should contain less than 10 000 ppm, better still less than 1 000 ppm total chlorine. For a total chlorine content in this order of magnitude, the risk of stress-cracking corrosion in the reactor can be controlled in industrially implemented processes. Although the aforementioned leaflet considers a chlorine content of 200 ppm to be critical, in industrial chemical processes the organophosphorus compound is only used in catalytic amounts, meaning that the total chlorine content in the reactor is, on account of dilution by the reactants, significantly less than 200 ppm, if the degree of contamination of the ligands used is within the desired range.
The chloride content can be determined analytically in a simple manner, for example by aqueous titration. A more extensive determination is that of the total chlorine content, which, as well as the chlorides, also encompasses chlorine bound in other forms. Emphasis on the total chlorine content is also of material relevance, in that it cannot be ruled out that chlorine bound in another form is also able to damage the reactor. In judging the limits for total chlorine, however, the chloride fraction remains crucial.
A suitable method for determining the total chlorine content is the combustion according to Wickbold with sample preparation to DIN 51408 and analysis by ion chromatography to DIN EN ISO 10304.
The patent literature discloses various methods for reducing the total chlorine content of organophosphorus ligands after the actual synthesis:
EP 0 285 136 claims a process for purifying tertiary organophosphites of pentavalent organophosphorus compounds which form as by-products of the synthesis or else as degradation or hydrolysis products of the tertiary organophosphites. The process envisages the treatment of the dissolved contaminated organophosphite with water at elevated temperature in the presence of a Lewis base. Lewis bases used are inorganic salts (carbonates, hydroxides, oxides), tertiary amines and polymers which carry amine groups.
One disadvantage of this process lies in the treatment with water. Not only the contaminants to be removed but also the tertiary organophosphites themselves react under the conditions specified, such that a portion of the product of value is lost according to the hydrolysis stability of the organophosphites.
DE 10 2004 049 339 describes a process for purifying phosphorus-containing chelate ligands by means of extraction using a polar extractant. The crude ligand was extracted here six times with a polar solvent, and then has a content of amine base, amine hydrochloride or mixtures thereof of less than 100 ppm. In this type of purification, however, enormous amounts of solvent are needed, which is in need of improvement from an economic and ecological point of view.
CN 101684130 A discloses the purification of phosphite ligands through the addition of deionized water and subsequent extraction. The organic solvent is removed by distillation in a subsequent step, and the crude product is recrystallized again. In this way, it was possible to obtain a product having a residual chlorine content of 0.01% by weight of chlorine.
In order to reduce the chlorine content of the ligand by this method, an extraction and a subsequent recrystallization are thus necessary. This means that a large amount of solvent has to be used, and yield losses because of the various purification steps and the possible lack of hydrolysis stability of the organophosphites cause a portion of the product of value to be lost.
WO 2012 095253 describes a process for preparing 6,6′-[(3,3′-di-tert-butyl-5,5′-dimethoxy-1,1′-biphenyl-2,2′diyl)bis(oxy)]bis(dibenzo[d,f][1,3,2]dioxaphosphepin) (termed: “biphephos”). The purification of the ligand takes place by repeated washing with various solvents.
Besides washing, distillation and recrystallization, in the course of the preparation of organophosphorus compounds there is also the option to purify the contaminated organophosphorus product with the help of filtration:
For example, EP2091958B1 describes the preparation of bisphosphites which are obtained dissolved in toluene and are filtered. This results merely in a partial removal of the chlorine, which is in need of improvement with regard to the intended use of the prepared organophosphorus compound as ligand in the hydroformylation of olefins.
It is known from EP1097936B1 to purify an organophosphorus product intended as stabilizer and dissolved in an organic solvent by adding water, a base and a solid drying agent to the solution in order to dry the organic phase. This produces two layers, namely one aqueous layer and one organic layer. The two layers are separated, the drying agent is filtered off from the organic phase and the solvent is evaporated. The chlorine-containing contaminants are thus eliminated via the aqueous phase. The disadvantage of this process is considered to be the addition of the water since the separation off of the aqueous phase in production on an industrial scale will not be possible with the required purity, meaning that residual water is left in the purified product. There, it is capable of decomposing the just obtained organophosphorus compound again by hydrolysis. Particularly then, if the organophosphorus product is intended for use as ligand in catalyst complexes, the presence of water is to be declined with regard to the hydrolysis.
Good chlorine values can be achieved by combining several purification steps: A combination of filtration and recrystallization for the purification of biphephos is shown in WO 2012/095255A1. In this process, a solid organophosphorus product is slurried in a solvent and filtered by means of a frit. The resulting, again solid organophosphorus product comprised, besides the desired biphephos, 2500 ppm of total chlorine. In order to reduce the total chlorine content, the solid was suspended in a solvent, heated and filtered. Then, the filtrate was recrystallized. The resulting purified organophosphorus product had a total chlorine content of only 35 ppm.
Although such a chlorine content is entirely satisfactory, this purification process requires many processing steps and large amounts of different solvents for the recrystallization, meaning that it is more suitable for the laboratory scale than for industrial ligand synthesis.